This invention relates generally to automatic test equipment, and more particularly, to a fault detecting arrangement for determining the presence of fault conditions on a relay bus in an automatic testing system.
A commercially available automatic test system (ATS) utilizes form A relays for interconnecting selected test points in a unit under test (UUT) to one or more measurement instruments. In this known ATS, a plurality of relays of the type wherein ohmic contacts are closed in response to an actuation signal are interconnected such that a first contact from each form A relay is connected to every other first contact of the other form A relays. In this manner, all such interconnected first contacts produce a relay bus.
The second contacts of the relays are connected to respective test points of the UUT, and other ones of such second contacts are connected to the inputs of respective test instruments, such as voltmeters, waveform analyzers, phase-angle voltmeters, counters, etc. A central controller for the ATS, which may be a digital computer, actuates one of the relays to form a connection between a test point in the UUT and a test instrument. Generally, the controller of the ATS is programmed to interconnect the test points to the test instruments in accordance with a predetermined sequence; the test instruments generally being of a programmable type so that the measurement readings can be recorded on a readout medium.
Automated test equipment of the above type suffers from several serious drawbacks. First, if a relay should fail in the closed state, catastrophic damage may result to the UUT and/or the test instruments. Upon the closure of a selected relay in a situation where an earlier closed relay has failed and remains closed, a short circuit across the test points would result through the relays and the relay bus. Clearly, the potential for damage to the UUT is obvious. However, such a short circuit condition may also overload and damage the next selected test instrument, and might also fuse the contacts of the more recently closed relay, thereby resulting in possible damage to the UUT, several relays, and a test instrument. Clearly, the closures of subsequent relays would cause rapidly escalating damage to the systems.
A further problem which is attributable to the use of relays in an ATS concerns the capacitance between the contacts of the relays which are interconnected in the relay bus arrangement. Such capacitance will allow charges to accumulate on the relay bus. This may produce errors in the subsequently measured values. In addition, the closure of a relay in a situation where the bus contains a substantial charge may result in a rapid discharge of the bus through the relay contacts, thereby causing damage to the relay and possible damage to the UUT.
The obvious solution to minimize the problem of fused relay contacts is to restructure the relay switching system to use known form C relays in a binary relay decoding tree configuration. This would assure that no ohmic contact path is created through the switching system from any test point to any other test point. However, this alternate configuration requires a substantially greater number of series relay contact closures to accomplish the equivalent test point to measurement instrument connection. As a result, this alternate configuration often does not provide an adequate low level signal interconnection path capability. Furthermore the use of multiple series relays would be less reliable and may introduce significant signal degradation thereby adversely affecting measurement quality. Finally this type of switching system requires the use of a greater number of larger and more costly relays, thereby significantly limiting package layout and flexibility.
It is, therefore, an object of this invention to provide a fault detection arrangement which prevents the closure of relays when an earlier closed relay has failed and remained closed in a relay bus switching system.
It is a further object of this invention to provide a fault detection arrangement wherein the closure of relays is prevented if the relay bus contains excess charge.